“Surround sound” is a term used in audio engineering to refer to sound reproduction systems that use multiple channels and speakers to provide a listener positioned between the speakers with a simulated placement of sound sources. Sound can be reproduced with a different delay and at different intensities through one or more of the speakers to “surround” the listener with sound sources and thereby create a more interesting or realistic listening experience.
Multi-channel surround sound is employed in movie theater and home theater applications. In one common configuration, the listener in a home theater is surrounded by five speakers instead of the two speakers used in traditional home stereo system. Of the five speakers, three are placed in the front of the room, with the remaining two surround speakers located to the rear or sides (THX dipolar) of the listening/viewing position. Among the various surround sound formats in use today, Dolby® Surround™ is the original surround format, developed in the early 1970's for movie theaters. Dolby® Digital™ made its debut in 1996 and is installed in more than 30,000 movie theaters and 31 million home-theater products. Dolby Digital is a digital format with six discrete audio channels and overcomes certain limitations of Dolby Surround which relies on a matrix system that combines four audio channels into two channels to be stored on the recording media. Dolby Digital is also called a 5.1 -channel format and was universally adopted several years ago for film-sound recording. Yet another new format is called Digital Theater System (DTS). DTS offers higher audio quality than Dolby Digital (1,411,200 versus 384,000 bits per second) as well as an optional 7.1 configuration.
The audio/video preamplifier (or A/V controller) handles the job of decoding the two-channel Dolby Surround, Dolby Digital, or DTS encoded signal into the respective separate channels. The A/V preamplifier output provides six line level signals for the left, center, right, left surround, right surround, and subwoofer channels, respectively. These separate outputs are fed to a multiple-channel power amplifier or as is the case with an integrated receiver, are internally amplified, to drive the home-theater loudspeaker system.
Manually setting up and fine-tuning the A/V preamplifier for best performance can be demanding. After connecting a home-theater system according to the owners' manuals, the preamplifier or receiver for the loudspeaker setup have to be configured. For example, the A/V receiver or preamplifier must know the loudspeaker type, so that the bass can be directed appropriately. For example, receivers may classify loudspeakers as “large” or “small”. Selecting a “small” loudspeaker will keep low-bass signals out of the speaker. This configuration is used when a subwoofer is used to reproduce low bass instead of the left and right speakers. If the system has no subwoofer and full-range left and right speakers, a “large” speaker setting should be selected. The setup may also require selecting “small” or “large” surround speakers. Next a center channel speaker mode (“normal” or “wide”) needs to be selected, as well as an appropriate center-channel delay so that the sound from all three front speakers arrives at a listener's ear at the same time. An additional short delay for the signal to the surround speakers of typically 20 ms may also have to be set to improve the apparent separation between front and rear sound.
In addition, the loudness of each of the audio channels (the actual number of channels being determined by the specific surround sound format in use) should be individually set to provide an overall balance in the volume from the loudspeakers. This process begins by producing a “test signal” in the form of noise sequentially from each speaker and adjusting the volume of each speaker independently at the listening/viewing position. The recommended tool for this task is the Sound Pressure Level (SPL) meter. This provides compensation for different loudspeaker sensitivities, listening-room acoustics, and loudspeaker placements. Other factors, such as an asymmetric listening space and/or angled viewing area, windows, archways and sloped ceilings, can make calibration much more complicated
It would therefore be desirable to provide a system and process that automatically calibrates a multiple channel sound system by adjusting the frequency response, amplitude response and time response of each audio channel. It is moreover desirable that the process can be performed during the normal operation of the surround sound system without disturbing the listener.